Process for the production of chromic acid



Sept. 29, 1936. J. w. Boss 1 PROCESS FOR THE PRODUCTION OF CHRONIC Filed Feb. 16,. 1955 ll 7///////////// I 2 "I l6 I l I o z r F Z i may I g l i w? 12 I IIR I 7 II 1 R!" 5 15 l I 4 I 5 I l I I l 4 /IIII/l/I/I/I/l Ill/IIII/I/I/I 7m, W floss Gnome ratus for the electrolytic production of chromic Patented Sept. 29," 1936 UNITED STATES PATENT? OFFICE PROCESS FOR THE PRODUCTION F CHROIVHC ACID I John Wesley Boss, Livingston, Mont. I Application February 16, 1935, Serial No. 6,904

@laims.

This invention relates to a process and appaacid and of caustic alkali.

In the prior art it is customary to produce chromic acid by'the action of an acid such as sulphuric on an alkali ehromate. or dichromate such as sodium chromate or dichromate. Separation of the chromic acid' is accomplished by crystallization and draining off the mother liquor containing in solution other products of the reaction. Y

Some of the disadvantagesof such a process are that diflicultyis encountered in removing other acid radicals from. the chromic acid, and

that the alkali originally combined as chromate or dichromate appears as a salt such as sodium sulphate. This salt is not of much value and. is

not suitable for use in preparing new quantities of sodium chromate.

I am aware that in the prior art, an electrolytic cell containing a diaphragm has been used for the purpose of converting alkali chromates to alkali dichromates. This process is. limited to the preparation 'of dichromates and employs a diaphragm. rendering the constructionoi the cell more complex and greatly increasing the elecacid than has the feed solution, the above described result to beattained without the use of a diaphragm.

A iurther object of the inventionisto produce chromic'acid free from other acid radicals and irom alkali salts.

A further object is to 'produce chromic acid electrolytically. An additional object is to revivii'y used' chromium plating solutions.

Another object of the invention is to provide an apparatus or multiple chamber cell'adapted to function with three different solutions and wherein means'are,fprovided for maintaining these solutions at their desired compositions.

The invention has for a further object to pro-' ride a multiple chamber cell wherein the planes of separation of the. solution may be observed. I With-the foregoing and other objects in view,

the'inventionwill be more fully described here- (01. cor-9) I inafter, and will be more particularly pointed out in the claims appended hereto.

In the drawing, wherein like symbols refer to like or corresponding parts throughout the several views,

Figure l'is a vertical transverse section taken through a cell constructed acording to the present invention substantially on the line i-i of Figurez and Figure 2 is a horizontal section taken through lltl the lower end of the cell on the line 2-2 oi Figure 1.

Referring now to the drawing which shows one form of the improved apparatus of this invention by'means of which the improved process oi this invention may be practiced, i0 designates the body of the cell which may be of substantially rectangular form and having appreciable depth and which may be made of lead, steel, stoneware, or other suitable material for containing chromic acid and caustic alkali. The cell is divided longitudinally by an intermediate-partition H extending from the upper end of the cell down to about two-thirds. or three-quarters of thecell depth and dividing the cell intoopposed chamhers i2 and E3 in the'upper part thereof. The

chambers 92 and I3 intercommunicate at their lower ends through a bottom chamber M. The cell in is provided in one or both sides with windows 95 which may comprise panes of glass or do other transparent material sealed over openings in the walls of the cell and which are disposed above the horizontal plane of the lower end of the partition ii. Of course these windows i5 may be located in other walls of the caller disposed at any other points of vantage for carrying out the purpose. The bottom chamber id of a the cell is open to a feed pipe l6 which may open at any suitable point into this chamber for supplying thereto feed solution. so

The cell body It! is provided at itsupper end. with overflow pipes H and it which communicate at their inner ends with the upper chambers it and- !3 respectively and at a desired height in the upper chamber to maintain a definite surface 4 level in said chambers.

The solution supply pipes 19- and 20 lead into the tops of the chambers l2 and 83 respectively.

for introducing thereto wateror for introducing into .each upper chamber a diluted solution of its product and communicating outside "the cell with any suitable source of supply. Each of the solution supply pipes i9 and 20 extends into its respective chamber not lower than the plane of jtheelectrode and is provided at its lower end with a return portion 2| to prevent disturbance when introducing the solutions' In each of the chambers l2 and I3 is suspended by any suitable means an electrode 22 and 23. The electrode 22 may be the anode and 23 the cathode of the cell, the anode 22 being preferably of lead or antimonial lead and the cathode 23 being of iron, steel or copper. The electrodes 22 and 23 may be connected to any suitable source of direct current for operating the cell.

The process or method of the present invention, set forth and described in connection with.

the above specifically described embodiment of the cell of this invention, is as follows:

In the following description, sodium chromate is used as an example but it is to be understood that other alkali salts of chromic acid or spent chromic acid solutions may be' used. A sodium 1 chromate solution having for example, a specific gravity of 1.400 is introduced into the bottom chamber l4 through the feed pipe IE to a surface level above the horizontal plane of the lower edge of the partition I I and within the line of vision of the windows I5. When making pure chromic acid, acid radicals other than the chromate radical should be excluded from this solution. In the anode chamber I2 is floated a solution of chromic acid having a specific gravity materially less than the feed solution, for example 1.200. The solution supply pipe IB is used to accomplish this floating in order to maintainthe plane of separation between the anode solution and the feed solution. Similarly a caustic soda solution of approximately the same specific gravity is floated in the cathode chamber being introduced through the solution supply pipe 2!].

The cell is now placed in operation by connecting the electrodes 22 and 23 to the proper poles of a source of direct current. Passage of current through the cell is necessary for the maintenance of the planes of separation. Stoppage of current supply is followed by diffusion across and eventual disappearance of both planes of separation. In cells constructed of metal the supply voltage should not be high enough to cause passage of current through the walls of the cell itself. About 4.5 volts can usually be used with good results. Warming of the cell is beneficial in decreasing the resistance to be overcome. In operation of the cell,

' passage of current causes chromic acid together with a small quantity of basic ions to accumulate in the layer floating in the anode chamber and caustic soda together with unused sodium chromate in the floating layer in the cathode chamber. As electrolysis proceeds the plane of separation at the top. of the feed solution may be observed through the windows IE to move downward in both chambers. This movement is to be balanced by the introduction of additional feed solution through the feed pipe l6. 7

The solution in the anode chamber increases in volume and density during the operation of the cell. The-increase in volume is kept down by means of the overflow pipe II. The density of the solution is maintained at the standard by the introduction of water or diluted overflow through the solution supply pipe IS. The overflow consists in the chromic acid solution produced by the cell.

Likewise in the cathode chamber l3 the volume is controlled by the overflow pipe l8 and chamber the overflow solution consists in caustic soda and sodium chromate.

The chromic acid produced as described may not be satisfactory for use in a plating bath. Therefore I call this solution crude chromic acid and it may be refined by passing through a second cell identical with the first. This crude solution may be evaporated to reduce the volume and increase the specific gravity and then fed through the feed pipe l6 of the second cell. The floating layers are maintained as in operation of the first cell. The acid overflow from the tube I! will now be found to be a nearly pure chromic acid solution suitable for use in chromium plating baths. Overflow from the cathode will be comparatively richer in sodium chromate and poorer in caustic soda than is the case of a cell fed with sodium chromate solution. This last overflow may be reworked in a cell using a sodium chromate feed to produce crude chromic acid an strong caustic solution.

To revive chromium plating baths that have become inoperative because of an accumulation of basic ions, the solution to be revived is fed through a feed pipe l6 and the cell operated as described for refining crude chromic acid.

Basic ions present in used plating solutions accumulate in the floating layer in the cathode chamber and any insoluble hydroxides or oxides will appear as a suspension or sludge. Working of the cell is benefited by maintaining a fairly rapid flow of solution through the solution supply tube 20 and out of overflow tube l8. In this way the insoluble hydroxides or oxides are washed out of the cell. Clear red solution will be discharged irom overflow tube ll as distinguished from the usual dark color of used plating solutions. The product discharged from overflow tube I1 when made up to its original volume and CIOs content will be found to be restored in its plating eificiency.

In some methods of cell construction it is not practical to provide two observation windows as shown. Normal operation of such a cell can be obtained with the use of only one observation window if the following conditions are fulfilled. Overflow tubes I1 and I8 should be at the same level and the overflows from such tubes be maintained at the same or nearly the same specific gravity, variable for example between the limits 1.190 and 1.210. It will be evident that if the specific gravities of the solutions in each chamber are equal then the planes of separation will be at the same level. If the cell is constructed of transparent material windows are not necessary.

A convenient way of assuring separation as before described is to provide a flow of solution through each upper compartment such that the.

rise of specific gravity is not disturbing, for example the solution feed may have a specific gravity of 1.190 and the overflow a specific gravity of 1.200.

It must be noted that in operation of the cell the plane of separation of the feed solution and upper chamber solutions should be kept at a point below the level of the electrodes 22 and 23 and above the plane of the lower edge of the partition ll The cell will not operate if the plane of sepa'rationrises above the electrodes or yet if the two upper chamber solutions intermix below the partition l I. The windows l5 are so positioned that when the.planes of separation are kept within their line of vision, these adverse conditions are prevented.

It will be obvious that various changes and modifications may be made in the apparatus and process above described without departing from the spirit of this invention and limited only by the scope of the following claims:

I claim:

1. In an electrolytic process for the production of chromic acid, continuously providing a feed solution of a. soluble salt of chromium capable of producing chromic acid, floating in separate cells above said feed solution product solutions one of alkali and the other of chromic acid said solutions of less density than the feed solution, maintaining a predetermined plane of separation between the feed and product solutions, passing an electric current through the solutions and drawing off the product solutions as an independent of the upper chambers in floating contact with the feed solution, maintaining a. second product solution of caustic alkali in the other product chamber and in floating contact with feed solution, passing a current between the product solutions and through the feed solution, and independently drawing off the product solutions from the chambers.

3. In an electrolytic process for the production of chromic acid, continuously providing. a feed solution containing a soluble salt of chromium capable of producing chromic acid, floating thereon on opposite sides of an impermeable partition separate product solutions one of alkali and the other of chromic acid, said product solutions having a specific gravity less than that of the feed solution, and passing an electric current between the product solutions and through the feed. solution by reason of the feed solution being common beneath the two product solutions.

4. In an electrolytic process for production of chromic acid and caustic alkali, providing a feed solution of sodium chromate of specific gravity around 1.400, floating on said feed solution in one cell compartment a chromic acid solution of spe 'cific gravity around 1.200, floating also on said feed solution in another cell compartment a sodium hydroxide solution of specific gravity around 1.200, and passing current between the chromic acid solution and the sodium hydroxide solution by way of the sodium chromate solution.

, 5. In an electrolytic process for the production JOHN WESLEY Boss. 

